In wireless radio frequency (RF) communications networks, multiple users communicate using a number of specific radio channels. FIG. 1 of the accompanying drawings illustrates a part of RF communications network 1 that includes network resources 3 which communicate with mobile terminals 5, using an air interface 7. The air interface 7 defines a number of radio channels for enabling transfer of data between the network resources 3 and the mobile terminals 5. Transmissions from the network resources 3 to the user 5 are ‘downlink’ communications, whilst transmissions from the mobile terminals 5 to the network resources 3 are ‘uplink’ communications. It will be readily appreciated that the term “network resources” is intended to encompass any suitable equipment in the mobile telecommunications network. For example, the network resources may be provided by a base station, a radio network controller (RNC) a network server, or any combination of these and other units.
When the network supports multiple users, it is necessary to control transmissions on the radio channels to avoid signal and data loss. Downlink transmissions are relatively straightforward to control since the network resources 3 have all relevant information concerning the users which it is serving, and so can control transmissions to the users itself. However, uplink communications are more difficult to control, since an individual user does not have information concerning the other users that are in communication with the network resources 3. It is therefore desirable to provide a technique that controls transmissions by the mobile terminals 5.
A Multiple Access Control (MAC) protocol is such a technique. Desired characteristics of a MAC protocol include low delay and high aggregate throughput or capacity. Previously considered MAC protocols can be divided into two groups: conflict-free, or ‘scheduled’ protocols, and contention-based protocols. Conflict-free protocols ensure that transmissions do not interfere with one another. With contention-based protocols collisions between transmissions can occur, and principles for resolving such conflicts must be defined.
Conflict-free protocols typically involve some signalling before data is transmitted to ensure that the transmission will not conflict with other transmissions. Although the duration of this signalling phase may be short, the delay may represent large fractions of the total transmission time, especially for transmission of small amounts of data. For large amounts of data the duration of the signalling phase is of less importance. However, since many data transmissions may be short, the signalling delay can be significant. One benefit of conflict-free protocols is that full medium usage can be achieved. This results in a high capacity potential.
Contention-based protocols allow direct transmission attempts, without previous signalling to ensure that the medium is free. This leads to very low delays. However, the risk of collisions between transmission attempts increases as load increases. Such collisions increase possible delays. Collisions also lead to the radio channels being frequency occupied by non-successful transmission attempts, which in turn results in poor aggregate throughput.
For low traffic loads the contention-based protocols generally yield lower delays, whereas for high traffic loads, the conflict-free protocols are better. Recent developments aim at combining the positive properties of both types of schemes by using two modes, i.e. contention-based transmission is used at low load and scheduled transmission is used for high load.
Currently work is ongoing to design the system concepts for a long term evolution of WCDMA (wideband code division multiple access) networks. This work is also referred to as evolved UTRAN, or E-UTRAN. A strong candidate for the air interface in E-UTRAN is Orthogonal Frequency Division Multiple Access (OFDMA) for the downlink and FDMA (frequency division multiple access) with variable bandwidth for the uplink. In the following, reference will be made to OFDMA and E-UTRAN, but these references should not be construed at limiting.
The E-UTRAN uplink is likely to rely mainly on scheduling, i.e. the network allocates certain chunks to each user for uplink data transmission through scheduling grants transmitted in downlink. The network typically base the issued grants on received scheduling requests from the users. That means that a request phase is needed prior to uplink data transmission that creates some delay. Still, some ways to send at least scheduling requests without a prior request will be needed.